Some implantable medical device operations require both low-power and high-power functions. An example is an implantable cardioverter-defibrillator (ICD). An ICD monitors cardiac function in a patient using implanted electrodes that capture electrical signals generated by the heart. This monitoring function requires the operation of low power control and analysis circuitry which may include, for example, amplifiers, filters, analog-to-digital converting hardware and/or a microcontroller. The low power circuitry will use currents in the range of microamps at voltages that have been decreasing with improvements in digital and analog circuitry.
In an ICD, monitoring is performed in part to determine whether a malignant arrhythmia is occurring in a patient's heart. If a malignant arrhythmia is identified, high power circuitry is used to build up and then release a large amplitude stimulus to the patient. Such a stimulus may have a voltage of hundreds or even thousands of volts, with total power ranging from less than a Joule up to 80 or more Joules.
A challenge in designing an implantable medical device (IMD) is to find power supply circuitry which can meet the low and high power needs of the device. The simplest approach is to use a single battery. However, challenges arise because available batteries often come in one of two forms: batteries capable of delivering large currents (i.e., high power outputs) but lacking optimal energy density, and batteries having high energy density but which encounter large internal impedances at high currents. One approach could be to provide separate low and high power sources using separate batteries, one having high current output capacity and one having high energy density. However, using separate batteries would cause the device to be disabled as soon as one of the batteries reaches its end-of-life. Alternative solutions are desired.